compensatory effects of overstory and understory

Compensatory effects of overstory and understory competition attenuate conifer

seedling growth responses in 40- to 70-year-old Douglas-fir stands

Timothy B. Harrington, USDA Forest Service, PNW Research Station, Olympia, WA

Douglas-fir seedling growing in the absence of overstory competition

Previous research:

• Douglas-fir seedlings require 40% of full sunlight to survive, but maximum development occurs under full sun (Mailly and Kimmins1977, Drever and Lertzman 2001)

• Western redcedar seedlings require only 10% of full sunlight to survive, and their maximum development occurs at 30% of full sunlight (Wang et al. 1994, Drever and Lertzman 2001)

• Recent research to determine overstory effects in stands of western hemlock (Mitchell 2001) and Douglas-fir (Barg and Edmonds 1999, Brandeis 1999) indicated that availability of belowground resources differed little among overstory densities

• As a result of these and other findings, Mitchell (2001) concluded that light was the primary factor limiting growth of conifer seedlings

Objectives:

• Characterize relationships of conifer seedling growth to various levels of overstory and understory competition.

• Determine how differences in availability of light, soil water, and nitrogen facilitate observed responses.

Douglas-fir seedling growing in the presence of overstory competition

The experiment was conducted at the Overstory Density Study, Capitol Forest, Olympia, WA:

• wide range of overstorydensities established in summer 2000

• conifer seedlings planted in early 2001 at 3-m spacing

Special thanks to:

Site 3

Site 2Site 1

3 sites:• 40- to 70-yr-old D. fir• 180-550 m elev.• D. fir SI50 41 m• silty loam soils• 200 cm annual ppt.

Thinned stand

ShelterwoodClear-cut

3 levels of overstorycompetition were selected at each site (nine 1.5-ha plots)

18 seedlings per conifer species were randomly selected in buffer areas of each plot Douglas-fir

(Pseudotsugamenziesii)

western hemlock (Tsuga heterophylla)western redcedar (Thuja plicata)

6 seedlings per species and plot were randomly assigned 1 of 3 levels of understory competition, defined by area of vegetation control (AVC):

3 msprayed areanon-sprayed area

3 m

conifer seedling

4.5 m2

9 m2

sprayed0 m2

0% AVC 50% AVC 100% AVC

100% AVC

50% AVC0% AVC

3 levels of understorycompetition (area of vegetation control)

Variables measured (2002-04):• seedling growth• overstory & understory covers• soil water content• D. fir needle nitrogen

vertical densitometer for overstory cover

hydrosense for soil water content

Experiment comprised a total of 162 seedlings per conifer species:

• 6 seedlings per understorylevel

• 3 understory levels• 3 overstory levels• 3 sites

Statistical analysis:• seedling=experimental unit• RCB with 3 blocks (sites)• split-plot (understory levels within overstory levels)• covariate: initial size• orthogonal polynomials to test for linear & quadratic effects

ANOVA for seedling growth:

161total146error AB

1initial size covariate4overstory x understory interaction

2understory competition level (B)4error A2overstory competition level (A)2blocksdfSource

field computer for data collection

Overstory density

clearcut shelterwood thinned

Ove

rsto

ry tr

ee c

over

(%)

0

10

20

30

40

50

60

70

20022004

Measurement year

BA (m2/ha) 2 12 26

• Overstory cover (light availability) varied systematically among the manipulated levels

0

20

40

60

80

100

0

1

2

3

50

100

Und

erst

ory

vege

tatio

n co

ver (

%)

Year

s sin

ce tr

eatm

ent

Area of veg. control (%)

Understory vegetation

Clearcut

ShelterwoodThinned

• Initially, understory cover varied among overstorylevels

• Later, it decreased linearly with AVC and was not related to overstory level

Douglas-fir responded only to specific combinations of overstory and understorycompetition:

• significant overstory x understory interaction

• small growth increase for shelterwood vs. thinned stands for all levels of AVC (attenuated response)

• superior performance in clear-cuts if AVC ≥ 50%

0

100

200

300

400

500

0

1

2

3

0

50

100

Stem

vol

ume

(cm

3 )

Year

s sin

ce tr

eatm

ent


Douglas-fir

Clearcut

Shelterwood

Thinned

Western hemlock responded to a range of overstory and understory densities:

• additive overstory and understory effects

• good performance in shelterwoods if AVC ≥50% (attenuated response)

• superior performance in clear-cuts if AVC ≥ 50%

0

300

600

900

1200

1500

0

1

2

3

0

50

100

Stem

vol

ume

(cm

3 )

Year

s sin

ce tr

eatm

ent


Western hemlock

Clearcut

Shelterwood

Thinned

Western redcedar responded primarily to reductions in understory density:

◦ marginal effects of overstory level

◦ growth peaked at AVC = 50% in shelterwoodstands0

20

40

60

80

100

0

1

2

3

0

50

100

Stem

vol

ume

(cm

3 )

Year

s sin

ce tr

eatm

ent


Western redcedar

Clearcut

Shelterwood

Thinned

Soil water

MonthApr Jul Oct Jan Apr Jul Oct Jan Apr Jul Oct Jan

Soil

wat

er c

onte

nt (%

)

10

20

30

0 50100

area of veg. control (%)

200420032002

• soil water during mid-summer increased linearly with AVC

• no relationship with overstorycompetition level

Area of vegetation control (%)0 50 100N

eedl

e N

con

cent

ratio

n (%

)

1.2

1.4

1.6

1.8

2.0

2.2

clearcutshelterwood

thinned

• greater in clear-cuts than in shelterwood or thinned stands

• increased linearly with AVC

• explained 78% of variation in 3rd-year seedling size

Douglas-fir needle nitrogen

Douglas-fir seedling size correlated with needle nitrogen

0

100

200

300

400

500

0

1

2

3

0

50

100

Stem

vol

ume

(cm

3 )

Year

s sin

ce tr

eatm

ent


Douglas-fir

Clearcut

Shelterwood

Thinned



eedl

e N

con

cent

ratio

n (%

)

1.2

1.4

1.6

1.8

2.0

2.2

clearcutshelterwood

thinned

0

100

200

300

400

500

0

1

2

3

0

50

100

Stem

vol

ume

(cm

3 )

Year

s sin

ce tr

eatm

ent


Douglas-fir

Clearcut

Shelterwood

Thinned



eedl

e N

con

cent

ratio

n (%

)

1.2

1.4

1.6

1.8

2.0

2.2

clearcutshelterwood

thinned

0

100

200

300

400

500

0

1

2

3

0

50

100

Stem

vol

ume

(cm

3 )

Year

s sin

ce tr

eatm

ent


Douglas-fir

Clearcut

Shelterwood

Thinned

Needle nitrogen concentration (%)1.4 1.6 1.8 2.0D. f

ir 3r

d-ye

ar v

olum

e (c

m3 )

0

100

200

300

400

500r2=0.78clear-cutshelterwoodthinned

Conclusions:

• Douglas-fir growth was intolerant of overstorycompetition, while hemlock and cedar demonstrated some tolerance

• All species responded to reductions in understorycompetition (Douglas-fir: only in clear-cuts)

• Overstory competition effects were determined primarily by differences in light and soil nitrogen

• Understory competition effects were determined primarily by differences in soil water and nitrogen

Any questions?

compensatory effects of overstory and understory

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